Optical measurements of patterned articles

ABSTRACT

A method and system are presented for use in measuring/inspecting a patterned article. Optical measurements are applied to a measurement site on the article by illuminating the measurement site with a plurality of wavelengths at substantially normal incidence of the illuminating light, detecting light returned from the illuminated site, and generating measured data indicative thereof. The measurements are applied to the measurement site through a polarizer rotatable between its different orientations selected from a number of pre-calibrated orientations.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 11/002,988, filed onDec. 3, 2004, now U.S. Pat. No. 7,330,259, issued on Feb. 12, 2008, theentire disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates in general to the field of opticalmeasurements/inspection, and particularly relates tomeasurement/inspection of patterned articles.

BACKGROUND OF THE INVENTION

It is known that polarized normal incidence spectrometry may be used formeasuring line profiles' parameters being applied to one-dimensional(1D) line arrays (pattern) on an article by help of RigorousCoupled-wave Theory (Further below RCWT) and the like modeling. The sameapproach can also be applied to two-dimensional (2D) line arrays or anyother 2D periodic pattern (e.g. arrays of lines, pads, vias, etc.).

U.S. Pat. No. 6,665,070 discloses a metrology device with a rotatablepolarizer that is calibrated to align the transmission axis of thepolarizer with the axis of orientation of a sample, such as adiffraction grating. The axis of orientation of the diffraction gratingcan be either the TE or TM axis. The system offset angle between thetransmission axis of the polarizer in its home position and an axis ofmotion of the stage, such as a polar coordinate stage, is determined.Whenever a new substrate is loaded onto the stage, the sample offsetangle between the axis of motion of the stage and the axis oforientation of a sample is measured. The polarizer offset angle, whichis the angle between transmission axis of the polarizer and the axis oforientation of the sample, is the sum of the system offset angle and thesample offset angle. Thus, by rotating the polarizer by an amountequivalent to the sum of the system offset angle and the sample offsetangle, the polarizer offset angle is reduced to zero. If desired, thepolarizer may be rotated once to compensate for the system offset angleand then rotated to compensate for the sample offset angle for eachnewly loaded substrate or the polarizer may be rotated to compensate forboth the system offset angle and the sample offset angle for each newlyloaded substrate.

U.S. Pat. No. 6,657,736 discloses a method and system for determining aline profile in a patterned structure, aimed at controlling a process ofmanufacture of the structure. The patterned structure comprises aplurality of different layers, the pattern in the structure being formedby patterned regions and un-patterned regions. At least first and secondmeasurements are carried out, each utilizing illumination of thestructure with a broad wavelengths band of incident light directed onthe structure at a certain angle of incidence, detection of spectralcharacteristics of light returned from the structure, and generation ofmeasured data representative thereof. The measured data obtained withthe first measurement is analyzed, and at least one parameter of thestructure is thereby determined. Then, this determined parameter isutilized, while analyzing the measured data obtained with the secondmeasurements enabling the determination of the profile of the structure.

According to the techniques of these patents, measurement of TE and TMpolarization components is used that require a certain specificorientation of the polarizer's axis relative to the orientation of theline arrays on a patterned article. Usually, the polarizer's axis isorientated parallel to the line (TE polarization measurement) and/orperpendicular to the line (TM measurements).

SUMMARY OF THE INVENTION

There is a need in the art to facilitate polarization-based measurementson patterned structures, by providing a novel measurement system andmethod.

The main idea of the present invention is associated with the following.As indicated above, the prior art techniques utilize measurements of TEand TM polarization components requiring a certain specific orientationof the polarizer's axis relative to the orientation of the line arrayson a patterned article. If an article under measurements is positionedon a movable X,Y-stage, it is sufficient to align the article relativeto coordinate system of the stage, so a pre-aligned polarizer can beused at two predefined positions relative to the stage coordinates. IfR-θ stage is used, it is necessary to align the polarizer to the articleon the stage in each orientation of the article. This technique suffersfrom drawbacks associated with the following:

Since an optical system induces some polarization and intensity change,it is necessary to calibrate the efficiency of the optical system foreach orientation of the polarizer. Since the time acceptable for suchcalibration is never infinite, practically calibration is carried out ata limited number of angles of polarization. Based on such a sampling,the efficiency of the optical system for any angle is calculated byinterpolating calibrated values at sampled angles, thus unavoidablyinducing an additional source of measurement error. Another drawback ofthe above technique relates to the optical axis deviation caused by thepolarizer rotation. If the optical axis of the polarizer (e.g.,Glan-Thompson prisms based polarizer) does not coincide perfectly withthe rotation axis (which is always the case), an optical beam thatpasses the polarizer is deflected and therefore impinges the measuredarticle slightly aside the nominal coordinates. Such a deflection is afunction of the angle of the polarizer's axis relative to itszero-position. It is thus clear that if the polarizer is to be alignedto an arbitrary positioned grating (the orientation of a patternedarticle), such a deflection should be pre-calibrated (with associatedinterpolation) and taken into account in positioning the article. Thisis another source of calibrating error that is caused by the polarizerrotation in the full range of possible orientations (±90°).

Yet another common drawback of the prior art measurement techniques isthat they do not exploit all the information that may be obtained fromthe sample, but use only the amplitude of diffraction efficiency for TEand TM polarization states. Valuable information about a phase shiftbetween these two polarization states in such case is ignored.

The present invention solves the above problems by measuring in apatterned article with at least three polarization states of incidentlight. This technique takes into account a phase difference between theTE and TM polarizations. It is known that this phase difference may beobtained by taking measurement in at least one additional orientation ofa polarizer relative to a grating. Usually, from the symmetry point ofview, a 45° angle is selected. Use of more angles does not provideindependent measurements but may be useful for reducing measurementerrors. So, by measuring with at least three polarization states, it ispossible to measure both the amplitude of the diffraction efficiency foreach polarization state and the phase shift between the polarizationstates.

There is thus provided according to one aspect of the invention, amethod for use in measuring/inspecting a patterned article, the methodcomprising applying optical measurements to a measurement site on thearticle by illuminating the measurement site with a plurality ofwavelengths at substantially normal incidence of the illuminating light,detecting light returned from the illuminated site, and generatingmeasured data indicative thereof, wherein the measurements are appliedto the measurement site through a polarizer rotatable between itsdifferent orientations selected from a number of pre-calibratedorientations.

The method utilizes calibration data indicative of an angularorientation (p of the pattern on the article under measurements and theselected polarization.

Preferably, these three selected polarization states include twomutually perpendicular polarizations corresponding to the angularorientations φ and (90+φ), and at least one intermediate polarizationstate corresponding to the polarization vector orientation in betweenthese mutually perpendicular polarization states. The intermediatepolarization may for example correspond to a 45° angular orientation.Considering four selected polarizations, the three polarization statesare selected as those corresponding to the orientations closest to thegiven orientation of the pattern on the article under measurements.

The measured data is processed to determine diffraction efficienciesR_(TE) and R_(TM) for, respectively, linear polarization states of theilluminating light relative to the pattern on the article undermeasurements, and a phase shift between them, thereby enablingcalculation of a diffraction efficiency R(φ) for any angular orientationφ of the polarizer relative to the pattern.

According to another aspect of the invention there is provided a methodfor use in measuring/inspecting a patterned article, the methodcomprising applying optical measurements to a measurement site on thearticle by illuminating the measurement site with a plurality ofwavelengths at substantially normal incidence of the illuminating light,detecting light returned from the illuminated site, and generatingmeasured data indicative thereof, wherein the measurements are appliedto the measurement site through a polarizer, rotatable between itsdifferent orientations, with at least three different orientations ofthe polarizer selected from at least four pre-calibrated orientations ofthe polarizer.

According to yet another aspect of the invention, there is provided amethod for use in measuring/inspecting a patterned article, the methodcomprising applying optical measurements to a measurement site on thearticle by illuminating the measurement site with a plurality ofwavelengths at substantially normal incidence of the illuminating light,detecting light returned from the illuminated site, and generatingmeasured data indicative thereof, wherein the measurements are appliedto the measurement site through a polarizer rotatable between itsdifferent orientations selected from a number of pre-calibratedorientations, data being provided about an angular orientation a of thepattern on the article under measurements with respect to a selectedzero position of the polarizer.

According to yet another aspect of the invention, there is provided asystem for use in measuring/inspecting a patterned article, the systemcomprising:

-   -   an illumination assembly configured and operated for producing        illuminating light of a plurality of wavelengths;    -   a detection assembly comprising a spectrometer for receiving        light of different wavelengths and generating measured data        indicative thereof;    -   a light directing assembly configured for directing the        illuminating light onto the article along an axis substantially        perpendicular to the article and collecting light returned from        the article along said axis and directing the collected light to        the detection assembly, the light directing assembly comprising        a polarizer assembly operated to be rotatable between its        different orientations; and    -   a control unit operable for receiving and processing the        measured data, the control unit utilizing calibrated data        indicative of an angular orientation (p of the pattern on the        article under measurements and at least three pre-calibrated        orientations of the polarizer, to thereby select the different        polarizer orientations for measurements on each measurement        site.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 is a schematic illustration of an optical system suitable to beused in the present invention;

FIG. 2 illustrates the principles of the present invention;

FIG. 3 exemplifies a calibration method of the invention for creatingreference data indicative of an optimal set of angles from which anglesfor measurements are to be selected;

FIGS. 4A-4B and 5A-5B exemplify the technique of the present inventionfor selecting three polarizer angles for measurements from four optimalangles;

FIG. 6 graphically exemplifies the principles of the present inventionfor selecting three optimal angular orientations of a polarizer formeasurements in each measurement site of the article;

FIG. 7 illustrates a flow diagram of the main operational steps in amethod of the present invention;

FIGS. 8 and 9 illustrate flow diagrams of two examples, respectively, ofthe method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is schematically illustrated an opticalsystem 10 for inspecting/measuring in a patterned article such assemiconductor wafer W actually presenting a grating (e.g., array oflines) on a substrate. The wafer is located on a stage 11, which ispreferably configured for rotation and back and forward movement alongat least one axis (the R-Theta stage).

The system 10 is configured as a normal incidence polarized reflectancespectrometer, and includes a light source assembly 12; a detectorassembly 14 including a spectrometer; a polarizer assembly 16 associatedwith a drive 18 that is operated to vary the orientation of thepreferred polarization of the polarizer with respect to the orientationof the grating; a light directing assembly 20; and a control unit 22that receives and interprets measured data coming from the detectorassembly. The light source assembly 12 includes at least one lightemitting element 12A and optionally includes a condenser lensarrangement 12B, as well as a fiber 12C through which light may beconveyed from the remote light emitter element(s). The light sourceassembly is configured for producing light of different wavelengths,which may be achieved by using different light emitting elements or abroadband illuminator. The light directing assembly 20 includes a beamsplitter 24 accommodated in an optical path of light coming from thelight source 12 to reflect it towards the wafer W and in the opticalpath of reflected light coming from the wafer to transmit the reflectedlight towards the detector 14; a focusing/collecting lens arrangement26; a pinhole mirror 28 for transmitting a part of collected light tothe detector 14 via a relay lens 29 and possibly another mirror 30 andfor reflecting the other part of the collected light towards an imagingassembly 32 via an imaging lens 33. The polarizer 16 is operated by thecontrol unit 22 to provide predetermined orientation of the preferredplane of polarization.

It should be noted that the pinhole mirror 28 works as a beam separatorbetween the measurement path propagating toward the detector 14 and animaging path propagating toward the imaging assembly 32 (e.g. CCDcamera); however other beam separator may be used instead of the pinholemirror, for example a beam splitter, switching mirror, etc.

Reference is made to FIG. 2 illustrating the principles of the presentinvention. Generally, during the measurements the polarizer ispositioned with any angular orientation φ_(i) relative to the gratingunder measurements. Considering β₁ as the zero-angle polarizationposition (β₁=0), φ=α, wherein α is an angle between the line array axis(grating) and the zero-angle polarizer position β₁. In general case,when β₁≠0, the angular orientation φ_(i) relative to the grating undermeasurements could be defined as φ_(i)=β_(i)−α. Diffraction efficiencyR(φ) for any angular orientation φ_(i) can be calculated by thefollowing equation:

R(φ)=R _(TE)·cos⁴ φ+R _(TM)·sin⁴φ+2 √{square root over (R _(TE) ·R_(TM))}·cos δ·sin²φ·cos²φ  (1)

In order to determine three independent parameters—R_(TE), R_(TM) andcos δ (phase shift between TE and TM polarizations), that should becompared to the theoretical calculated data, it is needed to measurediffraction efficiency on at least three different angles, φ₁, φ₂ andφ₃, corresponding to three fixed polarizer positions β₁, β₂ and β₃ andto solve the following system of equations:

R(φ₁)=R _(TE)·cos⁴φ₁ +R _(TM)·sin⁴φ₁+2√{square root over (R_(TE) ·R_(TM))}·cos δ·sin²φ₁·cos²φ₁

R(φ₂)=R _(TE)·cos⁴φ₂ +R _(TM)·sin⁴φ₂2√{square root over (R_(TE) ·R_(TM))}·cos δ·sin²φ₂·cos²φ₂

R(φ₃)=R _(TE)·cos⁴φ₃ +R _(TM)·sin⁴φ₃2√{square root over (R_(TE) ·R_(TM))}·cos δ·sin²φ₃·cos²φ₃   (2)

which may be rewritten in matrix form as:

$\begin{matrix}{\begin{pmatrix}{R\left( \phi_{1} \right)} \\{R\left( \phi_{2} \right)} \\{R\left( \phi_{3} \right)}\end{pmatrix} = {\underset{\underset{A}{}}{\begin{pmatrix}{\cos^{4}\phi_{1}} & {\sin^{4}\phi_{1}} & {\sin^{2}\phi_{1}} & {\cos^{2}\phi_{1}} \\{\cos^{4}\phi_{2}} & {\sin^{4}\phi_{2}} & {\sin^{2}\phi_{2}} & {{\cos^{2}\phi_{2}}\;} \\{\cos^{4}\phi_{3}} & {\sin^{4}\phi_{3}} & {\sin^{2}\phi_{3}} & {\cos^{2}\phi_{3}}\end{pmatrix}}\begin{pmatrix}R_{TE} \\R_{TM} \\{2\sqrt{R_{TE}R_{TM}}\cos \; \delta}\end{pmatrix}}} & (3)\end{matrix}$

wherein the matrix of coefficients will be denoted by A.

Here, A is a three-by-three matrix, which depends on three polarizerorientations selected for measurements.

In these linear equations, the unknowns are R_(TE), R_(TM) and cos δ,while the known parameters are φ_(1,2,3,) and R(φ_(1,2,3)). Afterdetermining the unknown parameters, they are compared to a theoreticalmodel, as will be described further below with reference to FIG. 8.Alternatively, the output of the theoretical model is first transformedinto R(φ_(1,2,3),) and then compared to the measurements, as will bedescribed further below with reference to FIG. 9.

So, it is possible, in principle, to keep an actual measurement positionβ_(i) of the polarizer constant, e.g., 0°, 90° or 45°, for determiningthe diffraction efficiency R for each orientation φ_(i) of the patternedarticle and thus to allow calibration of the polarizer only at threemeasurement angles avoiding a need for the calibration datainterpolation.

There is a special case when the grating is oriented at a 45°-anglerelative to the polarizer's axis (φ=45°), and the diffractionefficiencies R₀ and R₉₀ are actually the same values. Hence, instead ofthree independent different measurement conditions, only two are unique.This problem can be overcome by calibrating the polarizer for at leastone additional angle, e.g., (−45′), and thus four polarization stateswill be available for measurements: 0°, 90°, 45° and −45°. It is thusclear that even with four pre-selected angles, there might be gratingangles in which the matrix of coefficients A has a largecondition-number (defined as the ratio of its largest eigenvalue to itslowest eigenvalue, both in absolute values), indicating a high degree ofnon-uniqueness. The inventors have found that for the set of measurementangles β{−45°, 0°, 45°, 90°}, this occurs for angles of the form 22.5+45n where n=0, 1, 2, . . .

From the standpoint of uniqueness of three selected measurement angles,the use of more than three pre-calibrated angles β_(i) is preferred, andthus the higher uniqueness of the selected three measurements (at β₁, β₂and β₃) can be achieved. For simplicity, let's consider that angles Care accurate multiples of 15°, e.g., 0°, 150, 30°, 45° and so on, until180°. A simple algorithm for selecting these three angles from thepre-calibrated orientations is as follows:

-   -   β₁ is selected as the closest angle to the actual angle φ of the        array;    -   β₂ is selected as β₂=β₁+45°;    -   β₃ is selected as β₃=β₁+90°

If limiting the measurements to only four pre-selected angles isconsidered, it can be shown that the smallest condition number of thematrix A is achieved for the set of angles {0°, 65.2°, 90.4°, 116.9°}.For simplicity, such a set may be rounded to the following angles: {0°,60°, 90°, 120°}. Since matrix coefficients do not depend on application,this set of angles is optimal for any application.

Referring to FIG. 3, there is exemplified a flowchart of a method of theinvention showing the way of selecting the best set of N angles (for anynumber larger than 3) that can be found systematically. It makes use ofthe condition number CN (ratio of the largest eigenvalue and smallesteigenvalue) of the coefficients matrix A, defined above. Initially, afirst, arbitrary set of N angles of polarizer orientation is selectedβ₁, . . . , β_(N). All possible M combinations of 3 angles (triplets)from these N angles are defined, (triplet)₁, . . . (triplet)_(M). Then,for each grating angle α from a certain variety of K grating angles, α₁,. . . , α_(K) the triplet that provides the best (minimal) CN value isfound, namely CA⁽¹⁾ _(best), . . . , CN^((K)) _(best). Then, the worstvalue of CN_(best), namely, the maximal value, is found to present themerit function, CN⁽¹⁾ _(set), for this first set of polarizerorientation angles. Then, a second set of N polarizer orientation anglesis defined, β₁, . . . , β_(N), by slightly varying the angles of thefirst set, and similarly, a merit function, CN⁽²⁾ _(set) for this secondset of polarizer orientation angles is determined. This procedure isrepeated Q times, and merit functions CN⁽¹⁾ _(set), . . . , CN^((Q))_(set) are found, which process actually presents searching for anN-dimensional space and characterizing this space by the minimal valueof CN^((min)) _(set) that has its corresponding set of N polarizerorientation angles. The set of N angles defining this space ofCN^((min)) _(set) presents the best set of angles for actualmeasurements on the article.

After the best set of N angles is found, the selection rules aredetermined for selecting 3 angles out of N for each possible gratingangle β_(i) which has been recorded during the above calibrationprocedure. Now, during the actual measurements with a given gratingangle α, the corresponding best triplet of polarizer orientation angleis used, utilizing the above reference data.

Reference is made to FIGS. 4A-4B and 5A-5B exemplifying the technique ofthe present invention for selecting three polarizer angles formeasurements from four optimal angles. FIG. 4A shows four graphs thatcorrespond to, respectively, the selected angular orientations β₁, β₂,β₃ and β₄. Thus, each orientation of the grating (x-axis) has its threeoptimal corresponding orientations of the polarizer (y-axis), at whichthe measurements are to be taken. FIG. 4B is a table showing the bestthree angles out of the four angles for every possible grating angle:

For simplicity, such a set may be rounded to the following angles: {0°,60°, 90°, 120°}. This is illustrated in FIGS. 5A-5B.

The result of selecting three polarizer angles for each angle α,according to the above algorithm, but for more than 4 selected polarizerangles is illustrated in FIG. 6. The figure shows an example of usingthree-angles selecting algorithm from pre-calibrated orientations: 0°,15°, 30°, etc. (i.e., the polarizer rotation step of 15°). This figureshows the angular orientation β of the polarizer (vertical axis) vs. theangular orientation of the grating, namely, the angle α between the linearray axis and the zero-angle polarizer position (horizontal axis).Thus, each orientation of the grating has its three optimalcorresponding orientations of the polarizer, β₁, β₂ and β₃, at which themeasurements are to be taken.

Referring to FIG. 7, there is illustrated a flow diagram of the mainsteps of a method of the present invention. Measurements are applied toa measurement site on the article using polarized broadband light(generally, a plurality of wavelengths) normal incident into thearticle. The measurements are taken with at least three differentorientations of a polarizer at which the system was previouslycalibrated. A light response of the measurement site is detected.Measured data indicative of the detected light is processed to determinethe diffraction efficiencies corresponding to the selected polarizationstates, thereby enabling determining the pattern parameters.

Examples of a method of the present invention for one measurement sitewill now be described with reference to FIGS. 8 and 9. The measurementsystem is calibrated for a pre-selected set of N polarizer orientations,which includes at least four polarizer angular orientations β. When thearticle (wafer) is loaded in the system, actual measurements are taken.In order to optimize the measurements, the following procedure iscarried out: a grating-containing (patterned) site is selected formeasurements; the grating orientation α(φ) is determined; and based onthis information the optimal three polarizer orientations β₁, β₂ and β₃are selected from the calibration set β₁, β₂, . . . β_(N) (the setselected to be the best one during the above-described calibrationprocedure aimed at determining the reference data). Grating angularorientations φ₁, φ₂ and φ₃ corresponding to the polarizer orientationsβ₁, β₂ and β₃ are defined based on information on orientation of theline array axis of the pattern. Preferably, angle φ₁=α being an anglebetween the line array axis and the zero-angle polarizer position β₁. Itshould be noted that this optimization procedure may be done off-line,e.g., as a recipe design step for semiconductor wafers. Measurements areapplied to the article with the three selected polarizer orientationsand diffraction efficiencies R^(meas)(φ₁), R^(meas)(φ₂) and R^(meas)(φ₃)are determined.

The further interpretation of the so-determined (measured) diffractionefficiencies R^(meas)(φ₁), R^(meas)(φ₂) and R^(meas)(φ₃) is based on asuitable theoretical analysis of light diffraction on gratings, forexample, based on RCWT. The standard RCWT equations are formulated so asto provide such output parameters as R_(TE), R_(TM) and δ(phase shift).The theoretical analysis utilizes a predefined model based on theparameters of pattern (e.g., profile, period, tilt, etc.).

It should be understood that a certain inconvenience is caused by thatthe measured parameters R(φ₁), R(φ₂) and R(φ₃) are different from theoutput parameters of the calculation which are usually R_(TE), R_(TM)and cos δ, because the mostly used data interpretation method is basedon iterative calculation of theoretical parameters and comparing themwith the measured parameters: when the measured and calculatedparameters are equal within predefined measurement errors, the opticalmodel used for calculating the theoretical output parameters providesthe required information about the line profile. Such inconvenience canbe avoided. The preferred way for this is to transform the calculatedparameters R_(TE), R_(TM) and cos δ for each orientation β using theequation (1) above. Since the calculation accuracy is practicallyunlimited, such transformation will not induce any additional errors.After such transformation, the theoretical output parametersR^(sim)(φ₁), R^(sim)(φ₂) and R^(sim)(φ₃) will be compared directly tothe measured parameters R^(meas)(φ₁), R^(meas)(φ₂) and R^(meas)(φ₃).

According to the example of FIG. 8, the data indicative of the optimizedpolarization orientations set is processed using coefficient matrix totransform R_(TE), R_(TM), and cos (δ) (determined using RCWT) toR^(sim)(φ₁), R^(sim)(φ₂) and R^(sim)(φ₃) by the equation (1) for each φ.Then, measured values R^(meas)(φ₁), R^(meas)(φ₂) and R^(meas)(φ₃) arecompared to R^(sim)(φ₁), R^(sim)(φ₂) and R^(sim)(φ₃). If these valuessatisfy a matching condition with a predefined accuracy, themeasurements are accepted and are continued for a next site on thearticle. If no matching is detected, the model parameters are changed tominimize the difference, and new values of R_(TE), R_(TM), and cos(δ)are determined.

There is another way of matching the measured and calculated parameters.

In the example of FIG. 9, the measured values R^(meas)(φ₁₎, R^(meas)(φ₂)and R^(meas)(φ₃) are processed using the matrix coefficients todetermined the corresponding values of R^(meas) _(TE), R^(meas) _(TM),and cos(δ)^(meas) by solving the equation (3). Then, the so-measuredvalues R^(meas) _(TE), R^(meas) _(TM), and cos(δ)^(meas) are compared tosimulated (using RCWT) values R^(sim) _(TE), R^(sim) _(TM), andcos(δ)^(sim). If the matching exists (with predefined accuracy), themeasurement is accepted; if not, the model parameters are changed andnew values of R^(sim) _(TE), R^(sim) _(TM), and cos(δ)^(sim) areobtained.

Those skilled in the art will readily appreciate that variousmodifications and changes can be applied to the embodiments of theinvention as hereinbefore exemplified without departing from its scopedefined in and by the appended claims.

1. A method for use in measuring/inspecting a patterned article, themethod comprising applying optical measurements to a measurement site onthe article by illuminating the measurement site with a plurality ofwavelengths at substantially normal incidence of the illuminating light,detecting light returned from the illuminated site, and generatingmeasured data indicative thereof, wherein the measurements are appliedto the measurement site through a polarizer rotatable between itsdifferent orientations selected from a number of pre-calibratedorientations; wherein the measurements applied to the measurement siteare carried out with the at least three different orientations of thepolarizer selected from the at least four pre-calibrated orientations;wherein said at least three selected polarization orientations includetwo mutually perpendicular polarizations corresponding to the angularorientations β and (90+β), and at least one intermediate polarizationorientation corresponding to the polarization vector orientation inbetween said mutually perpendicular polarization states.
 2. The methodof claim 1, wherein the intermediate polarizations include thatcorresponding to a (−45°) angular orientation.
 3. The method of claim 2,wherein the at least three selected polarization orientations includeangular orientations β₁, β₂ and β₃ spaced from each other by a certainstep.
 4. The method of claim 3, wherein said certain set of anglesincludes: {0°, 65.2°, 90.4°, 116.9°}.
 5. The method of claim 3, whereinsaid certain set of angles includes: {0°, 60°, 90°, 120°}.
 6. The methodof claim 2, wherein the selected polarization orientations include atleast one additional intermediate polarization orientation.
 7. Themethod of claim 6, wherein the three selected polarization orientationsinclude the polarization angular orientation closest to the givenangular orientation of the pattern on the article under measurementswith respect to a zero-angle of the polarization orientation.
 8. Themethod of claim 2, comprising processing the measured data anddetermining diffraction efficiencies R_(TE) and R_(TM) for,respectively, linear polarization states of the illuminating lightrelative to the pattern on the article under measurements, and a phaseshift between them, thereby enabling calculation of a diffractionefficiency R(φ) for any angular orientation φ of the polarizer relativeto the pattern.
 9. The method of claim 8, wherein the diffractionefficiency R(φ) is calculated as:R _(φ) =R _(TE)·cos⁴ φ+R _(TM)·sin⁴φ+2√{square root over (R _(TE) ·R_(TM))}·cos δ·sin²φ·cos²φ wherein φ is the angle between the grating andpolarizer orientations, R_(TE) and R_(TM) are the measured diffractionefficiencies for said mutually perpendicular polarization orientations,and δ is a phase shift between the diffraction efficiencies R_(TE) andR_(TM).
 10. The method of claim 9, comprising processing the measureddata to determined measured values of diffraction efficiencies,R^(meas)(φ₁), R^(meas)(φ₂) and R^(meas)(φ₃) for the three differentangular orientations β₁, β₂ and β₃.
 11. The method of claim 10,comprising using a predetermined theoretical model based on parametersof the pattern and defining matrix coefficients as follows:$\begin{pmatrix}{R\left( \phi_{1} \right)} \\{R\left( \phi_{2} \right)} \\{R\left( \phi_{3} \right)}\end{pmatrix} = {\begin{pmatrix}{\cos^{4}\phi_{1}} & {\sin^{4}\phi_{1}} & {\sin^{2}\phi_{1}} & {\cos^{2}\phi_{1}} \\{\cos^{4}\phi_{2}} & {\sin^{4}\phi_{2}} & {\sin^{2}\phi_{2}} & {\cos^{2}\phi_{2}} \\{\cos^{4}\phi_{3}} & {\sin^{4}\phi_{3}} & {\sin^{2}\phi_{3}} & {\cos^{2}\phi_{3}}\end{pmatrix}\begin{pmatrix}R_{TE} \\R_{TM} \\{2\sqrt{R_{TE}R_{TM}}\cos \; \delta}\end{pmatrix}}$ wherein R(φ_(1,2,3)) are the diffraction efficienciesfor the angular orientations φ_(1,2,3), to determine theoretical valuesof R_(TE), R_(TM) and cos δ.
 12. The method of claim 11, comprisingusing the matrix coefficients to transform the theoretical valuesR_(TE), T_(TM) and cos δ to theoretical values of diffractionefficiencies R^(sim) (φ₁), R^(sim) (φ₂) and R^(sim) (φ₃).
 13. The methodof claim 12, comprising comparing the measured values R^(meas)(φ₁),R^(meas) (φ₂) and R^(meas) (φ₃) to the corresponding theoretical valuesR^(sim)(φ₁), R^(sim)(φ₂) and R^(sim)(φ₃), to determine whether theysatisfy a matching condition with a pre-defined accuracy.
 14. The methodof claim 13, comprising varying the model parameters, upon detectingthat the matching condition is satisfied.
 15. The method of claim 14,comprising using inverse of the matrix coefficients to transform themeasured values R^(meas)(φ₁), R^(meas)(φ₂) and R^(meas)(φ₃) to measuredvalues R^(meas) _(TE), R^(meas) _(TM) and cos δ^(meas).
 16. The methodof claim 15, comprising comparing the measured values R^(meas) _(TE),R^(meas) _(TM) and cos δ^(meas) to the theoretical values of R_(TE),R_(TM) and cos δ, to determine whether they satisfy a matching conditionwith a pre-defined accuracy.
 17. The method of claim 16, comprisingvarying the model parameters, upon detecting that the matching conditionis satisfied.
 18. The method of claim 2, wherein the pattern is an arrayof spaced-apart parallel regions.
 19. The method of claim 18, whereinthe pattern acts as a diffractive grating for the illuminating light.20. A system for use in measuring/inspecting a patterned article, thesystem comprising: an illumination assembly configured and operated forproducing illuminating light of a plurality of wavelengths; a detectionassembly comprising a spectrometer for receiving light of differentwavelengths and generating measured data indicative thereof, a lightdirecting assembly configured for directing the illuminating light ontothe article along an axis substantially perpendicular to the article andcollecting light returned from the article along said axis and directingthe collected light to the detection assembly, the light directingassembly comprising a polarizer assembly operated to be rotatablebetween its different orientations; and a control unit operable forreceiving and processing the measured data, the control unit utilizingcalibrated data indicative of an angular orientation (p of the patternon the article under measurements and at least three pre-calibratedorientations of the polarizer, to thereby select the different polarizerorientations for measurements on each measurement site; wherein said atleast three selected polarization orientations include two mutuallyperpendicular polarizations corresponding to the angular orientations βand (90+β), and at least one intermediate polarization orientationcorresponding to the polarization vector orientation in between saidmutually perpendicular polarization states.